The use of liquid shim in structural joints is commonplace in the aircraft industry. Liquid shim compensates for small tolerance mismatches between components prior to fastening and avoids build stresses that can lead to delamination of composite materials, stress corrosion cracking of metallic parts and premature fatigue cracking.
Liquid shim is generally a two-part epoxy paste. It may be applied by injection into the joint or by a “butter and paste” application. The liquid shim cures in place in the joint and the composition is optimised for compression and fatigue properties.
Current processes require joints to be disassembled several times during manufacture for cleaning, de-burring and application of liquid sealant. An example process which would require the joint to be assembled three times would be as follows:—
Step 1—Assemble joint and measure the gap or use a decision gauge
Step 2—Disassemble the joint and clean the faying surfaces
Step 3—Mix the liquid shim
Step 4—Apply liquid shim and assemble the joint
Step 5—Allow the shim to cure
Step 6—Disassemble joint for cleaning, preparation and drilling
Step 7—Inspect the joint
Step 8—Re-work the shim as necessary
Step 9—Apply sealant and final fasten joint together.